1. Field of the Invention
This invention relates to an image recording apparatus for recording images on a recording medium by a light beam.
2. Description of the Prior Art
Various image recording apparatuses utilizing a light beam are known. For example, as a photosensitive medium of an electrophotographic recording apparatus using a laser as a light source, use has been made of selenium, a selenium type alloy, a cadmium sulfide resin dispersion system, a charge transfer complex of polyvinyl carbazole and trinitrofluorenone, or the like. Also, as the laser, use has been made of a gas laser such as helium-cadmium, argon, helium-neon or the like and further, compact low-cost semiconductor lasers which can be directly modulated have come to be used. The spectrum of such semiconductor lasers is a coherent one of very high monochromaticity as shown, for example, in FIG. 1 of the accompanying drawings.
As regards the photosensitive medium, attention has been paid to a multi-layer type photosensitive medium comprising a charge transfer layer and a charge producing layer so that a sufficient sensitivity and charging characteristic may be obtained in accordance with the wavelength of the laser light used. As compared with a photosensitive medium using a single photoconductive layer, in the multi-layer type photosensitive medium, photosensitivity can be caused to depend only on the charge producing layer and therefore, photoconductive materials having photosensitivity to the wavelength of the laser light used can be relatively freely chosen.
The charge producing layer of the multi-layer type photosensitive medium has the function of absorbing light and producing charges, and the thickness thereof is usually as thin as 0.1 to 5 .mu.m. The charge transfer layer has the function of receiving electrostatic charges and transporting charges, and a material hardly absorbing image forming light is used for such layer and the thickness thereof is usually 5 to 30 .mu.m.
Now, when an image is formed by the use of such multi-layer type photosensitive medium and by line-scanning a laser light by a laser beam printer, there is no problem in forming line images such as characters, but in the case of a solid image, a density irregularity of interference fringe appears.
The reason for this is considered to lie in the interference between the reflected light on the surface of the charge transfer layer and the reflected light on the surface of a substrate such as a metal. That is, the multi-layer type electrophotographic photosensitive medium is of a construction in which, as shown in FIG. 2 of the accompanying drawings, a charge producing layer 2 and a charge transfer layer 3 are laminated on a substrate 1. When a laser light 4 (whose light emission wavelength is about 0.8 .mu.m for a semiconductor laser and about 0.63 .mu.m for a helium-neon laser) is incident on such multi-layer type photosensitive medium, there occurs interference between a reflected light 5 on the surface of the charge transfer layer 3 of a great reflection factor and a light 6 reflected by the surface of the substrate 1 and coming out of the surface of the charge transfer layer 3, as shown in FIG. 3 of the accompanying drawings. If the refractive index of the charge producing layer 2 and the charge transfer layer 3 is n and the thickness thereof is d.sub.1 and the wavelength of the laser light is .lambda., when nd.sub.1 is a multiple as great as .lambda./2, the intensity of the reflected light is maximum, that is, the intensity of the light coming into the interior of the charge transfer layer 3 is minimum (due to the law of conservation of energy), and when nd.sub.1 is an odd number multiple as great as .lambda./4, the reflected light is minimum, that is, the light coming into the interior is maximum. Now, location irregularity of the order of 1 .mu.m is unavoidable for d.sub.1. Laser light is of good monochromaticity and coherent and therefore, the aforementioned interference condition changes correspondingly to the location irregularity of d.sub.1 and thus, location irregularity of the amount of absorption of the laser light in the charge producing layer 2 occurs. This factor is considered to appear as the interference-fringe-like irregularity of density of the solid image.
The density irregularity due to such interference fringe is remarkable, particulary when a multi-layer type photosensitive medium is used as described above, and also in other recording media, the density irregularity as mentioned above may sometimes occur depending on the structure thereof.